The present invention relates to an optical recording/reproducing apparatus for recording and reproducing an information signal by emitting a laser beam onto an optical disk medium, and more particularly to a novel optical recording/reproducing apparatus equipped with an automatic focus offset compensator.
An optical recording/reproducing apparatus records and reproduces information by means of an optical head which emits a laser beam onto an optical disk, such as an optomagnetic disk, and involves a tracking servo circuit for enabling the laser beam to properly trace a recording track on the disk and a focus servo circuit for accurately focusing the laser beam on a recording medium face of the disk.
A conventional focus servo circuit utilizing a conventional focus error detecting method such as the knife edge method has a drawback that the compensating range of the focus servo is usually very narrow, only between .+-. tens of microns. Therefore, according to a prior art optical recording/reproducing apparatus, since the offset of the focusing position is substantially varied over the compensating range of the conventional focus servo circuit by an excessive change of temperature or humidity or by mechanical vibration, compensation due to the offset may be impossible. This might invite a deterioration in the error rate of a reproduced signal and further make it impossible for the tracking servo to trace the track.